This invention relates to a fatliquoring composition and a process for treating leather with that fatliquoring composition.
The processing of leathers to form shoes with PUR or PVC soles directly sprayed on (i.e. without an additional adhesive bond between the sole and upper leather) has become increasingly significant in recent years, particularly in the field of sport shoes. In such processes a shoe upper is drawn over a (metal) last and the uppers are introduced into a relatively tightly closing mold. The sole is then sprayed on directly without additional bonding using a reactive polyurethane mixture or, optionally a PVC mixture. This eliminates the need for a bonding agent between upper and sole.
This process may be economically carried out when the leathers used are dressed with products which enter into a firm bond with polyurethanes under the processing conditions, (e.g., with PU-coated skivers). However, the direct spraying-on process presents difficulties when the leathers are to be naturally processed because, in most cases, the standard fatliquoring materials for leather are only superficially absorbed and therefore act like a release agent between the leather and the polyurethane. High adhesion, as required in particular for sports shoes, cannot be obtained because, under the heat effect generated during the spraying-on process, the polyurethane is unable to bind itself sufficiently firmly to the fatliquoring agent diffused onto the surface of the leather and to the leather itself.
One way of avoiding this effect is to treat the leather after fatliquoring with polycationic substances in order to fix the fatliquoring agent to the leather. However, this does not produce the desired result because the polycation-active substances are also deposited onto the leather surface (like pigments) together with the fatliquoring agents and reduce the adhesion of the PUR sole subsequently applied.
Other possibilities for improving adhesion between leather and polyurethane which have been tried include the use of fatliquoring agents which have an affinity for leather and are also capable of reacting with the isocyanate groups in the polyurethane. Although fatty acid derivatives of the type described, for example, in SU-PS No. 840,220 theoretically have these properties, these derivatives also act like a release agent due to their C.sub.10 -C.sub.16 fatty acid residues. The nonylphenol-based Mannich bases or alkoxylation products thereof (disclosed e.g., in U.S. Pat. No. 3,720,606) which in principle may be used in the same way, have a catalytic effect on other isocyanate reactions (for example, on the NCO--OH reactions or NCO trimerization) and do not improve adhesion either. The polyglycolether amines disclosed in German Offenlegungsschrift No. 2,539,671 show similar behavior.
Until now, the only successful processes for producing upper leathers of any type for the direct spraying-on of soles required mechanically roughening the leathers by abrasion before the spraying-on process or treating the leathers with an adhesion promoter before the PUR or PVC sole was sprayed on in order to strengthen adhesion between the leather and the polyurethane. Both processes involve additional and expensive operations.
For example, it is very difficult where the leather is mechanically roughened to confine the abrasion process to the area which will be in contact with the PUR sole. If the abrasion process exceeds those bounds, unattractive rough patches are formed on the upper leather. However, if the abrasion process falls short of those boundaries, adhesion between sole and upper is greatly reduced over the non-roughened areas.
The same problem of keeping exactly to predetermined limits also affects the second method of aftertreating the upper leather before the spraying-on process (i.e., applying a suitable primer or adhesion promoter) if unattractive streaks on the upper material on the one hand and reduced adhesion of the sole on the other hand are to be avoided. Primers of this type are, for exampe, aromatic oligourethanes which generally show a pronounced tendency towards yellowing.
Unfortunately, all such attempts to make upper leathers to which a PUR sole reliably adheres in the absence of the above-mentioned labor-intensive steps of roughening or pretreatment with the primer have failed.